The key step in the mechanism of the Palladium-catalyzed homocoupling of arylboronic acids ArB(OH)(2) (At = 4-Z-C6H4 with Z = MeO, H, CN) in the presence of dioxygen, leading to symmetrical biaryls, has been elucidated by using density functional theory. In particular, by starting from the peroxo complex O2PdL2 (L = PPh3 generated in the reaction of dioxygen with the Pd(0) catalyst, the fundamental role played by an intermediate formed by coordination of one oxygen atom of the peroxo complex to the oxophilic boron atom of the arylboronic acid has been pointed out. This adduct reacts with a second molecule of arylboronic acid to generate a cis-Ar-Pd(OOB(OH)(2))L-2 complex that can form the stable intermediate trans-Ar-Pd(OH)L-2 (experimentally characterized) through a sequence of hydrolysis and isomerization reactions. All theoretical insights are in agreement and do substantiate the experimentally postulated mechanism. Furthermore, direct comparison of experimental and computed spectroscopic parameters (here, (31)p chemical shifts) allows us to confirm the formation of the intermediate.